The present invention relates to a system and method for testing a disc, and more particularly to a system and method of precisely positioning a glide head above a disc during a glide height test.
As the density of data recorded on magnetic discs continues to increase, the flying height of magnetic transducers with respect to the disc must be reduced to accurately read and write information on the disc. As a result, the magnetic recording disc must accommodate the lower fly height of the transducer and the slider supporting it, meaning that the disc surface must be extremely smooth and uniform. In order to certify that a magnetic disc is adequately smooth for use in a disc drive system, glide height tests are performed on the disc.
In addition to the general requirement of reduced fly height, magnetoresistive (MR) heads are extremely sensitive to small physical defects in the surface of the disc, such as undulations on the disc surface and microscopic debris on the disc. When the MR head strikes a defect, there is a momentary frictional heating of the MR element, known as a thermal asperity. This heating effect increases the resistance of the MR head, which causes data errors and loss of information in reading the disc. Thus, it is important to ensure the surface of any disc is relatively free of defects which may adversely affect the ability of the MR head to function.
Thus, one of the final steps in manufacturing a disc is to perform a glide height test. In conducting a glide height test, a single disc is placed on a spin stand and the disc is spun at extremely high speeds, often approaching over 10,000 revolutions per minute (rpm). A glide head suspended on a suspension arm is moved across the surface of a disc as the disc is spun. A typical glide head often comprises a piezoelectric transducer mounted on an air bearing slider. During the glide height test, the glide head xe2x80x9cfliesxe2x80x9d over a disc surface at a predetermined height above the disc surface, known as the xe2x80x9cglide height.xe2x80x9d If contact occurs between the glide head and the disc, an asperity at least as large as the glide height has been detected. Thus, it is possible to test the surface of a disc for asperities by controlling the glide height based on the size of the defect the glide head is meant to detect.
If the glide head encounters an asperity during the test, the collision causes the glide head to vibrate and deform, which in turn causes the piezoelectric element to vibrate and deform. Such contact may cause many vibration modes of the piezoelectric element and slider, with each mode generating a voltage at its specific frequency. The signals generated by the piezoelectric element are fed to a pre-amplifier and a band pass filter. A digital data acquisition system on the glide tester then processes the filtered data, uses the data to determine whether the disc passes or fails the glide height test. Should the disc fail a glide height test, it is possible to use a burnishing head to attempt to smooth out surface asperities.
In establishing the height at which the glide head is flying above the disc, a well documented relationship between the speed of rotation of the disc and the fly height of the glide head due to the air bearing is used. The spacing between the glide head and the disc are likewise controlled using a speed sensitive glide head. This practice has been adequate for many generations of discs, but is becoming inadequate as fly heights are reduced to as small as 0.5 microns. In particular, problems with calibration and sensitivity make it difficult to ensure that the glide head is flying at precisely the desired fly height.
Thus, there is a need in the art for determining the fly height of glide head with increased precision.
To determine the fly height of glide head during glide height testing, the glide head is lowered until there is contact between the glide head and the disc surface. Once contact occurs, a control system uses the point of contact to establish a base line indicating the location of the disc surface. The glide height can be controlled relative to the base line so that the glide head is flown at precisely the desired glide height above the surface of the disc.
Once the base line is established, controlling the glide head so that it flies at the desired fly height above the base line can be achieved using many methods. For glide height testers using glide heads having piezoelectric transducers, the piezoelectric transducer can be actuated so that the glide head is moved to a desired clearance above the base line. Other embodiments for achieving the desired glide height involve utilizing well known relationships between the rotation speed of the disc and the air bearing surface to control the location of the glide head above the disc. It is also possible to use an electrical response of the glide head to bits recorded on the disc to control the glide head flight so that it flies at the desired glide height.